With the development of integrated circuits (ICs), electronic products tend to become more and more smaller in size, more and more intelligent, with a high performance and a high reliability. The smaller dimension and higher integration of IC present a higher requirement to the IC package technique.
The flip chip is a technique in which a chip is flipped over and then connected with a board by solder balls formed on the surface of the chip so as to reduce the package size. The flip chip technology may satisfy the requirement for high performance (such as, high speed, high frequency and smaller pins) with a smaller size. In this way, the product will have a good electric performance and a good heat-conductive property.
Bump fabrication is a key technique in flip-chip technology. The bump is a metal solder ball formed by depositing solder over an interconnection metal layer of a chip and reflowing at a certain temperature. The conventional processes for bump fabrication are shown in FIG. 1A to FIG. 1B. As shown in FIG. 1A, a chip 10 on which surface formed a passivation layer 11 and an interconnection metal layer 12 is provided. An under-bump metallurgy (UBM) layer is formed on the surface of the chip 10. Next, a photoresist layer is coated over the UBM layer, and is subjected to exposure and development so as to form a photoresisit opening. Next, solder for bump 16 is deposited over the photoresist opening. Then the photoresisit layer and part of the UBM layer 13 are removed. Referring to FIG. 1B, flux (not shown) is applied over the surface of the chip, and is reflowed at a certain temperature to form a bump 17. Then, the flux on the bump 17 and over the surface of the chip 10 is removed.
The typical flux consists of colophony resin, active agent, solvent, thixotropic agent and other additives. In the conventional reflow process, the flux is mainly used to remove contaminations on the solder surface, deoxidize metal oxides, and facilitate the reflow. In the conventional reflow process, a bump having a desired shape is formed by using the flux and controlling the temperature of the reflow. The process mainly includes the following stages: a) a pre-heating stage for activating the flux; b) an evaporating stage for evaporating some solvent in the flux and for out-gassing the bubbles in the solder; c) an initially-melting stage for further evaporating the solvent in the flux, in which the flux interacts with the impurities including oxide and metal salt on the solder surface so as to remove the impurities on the solder surface; d) a reflow stage for melting solder at a predetermined temperature; and e) a cooling stage for shaping the melted solder into a ball under its own surface tension.
Since the flux is expensive in price and a cleaning process is also required after bump formation, the above described process wastes the resources, and the cost of the solder bump formation is increased. Moreover, new defects can be introduced during the cleaning process because the flux may react with the flux removing material.
Therefore, a fluxless reflow for solder formation is emerging as a challenging and competitive process. A flip-chip method without flux in the reflow process is disclosed in the Chinese Patent Application No. 02810933, in which the chip and the corresponding substrate are aligned approximately by a contact pressure before bump reflowing, therefore the flux is not necessary for joining the chip to the substrate. However, the above method can only be used to the reflow in chip interconnection. Up to now, a fluxless reflow process for bump formation has not been presented.
The flux is used to deoxidize the metal oxide and cover the melted solder in a conventional reflow process. In a fluxless reflow process, bubbles may be caused by the evaporating of organic material in solder. As a result, the solder may be splashed and then form solder balls around the bumps during the reflow.